1. Field of the Invention
The invention generally relates to heating systems for semiconductor manufacturing equipment. More specifically, the invention relates to radiant heat sources for improving temperature uniformity.
2. Description of the Related Art
Chemical vapor deposition (CVD) is a very well known process in the semiconductor industry for forming thin films of materials on wafers. In a CVD process, gaseous molecules of the material to be deposited are supplied to wafers to form a thin film of that material on the wafers by chemical reaction. Such formed thin films may be polycrystalline, amorphous, or epitaxial. Typically, CVD processes are conducted at elevated temperatures in order to accelerate the chemical reaction.
In the semiconductor industry, it is important that the material be deposited with uniform properties, such as thickness and composition, over the wafer. In Very Large and Ultra Large Scale Integrated Circuit (VLSI and ULSI) technologies, the wafer is divided into individual chips having integrated circuits thereon. If a CVD process step produces deposited layers with non-uniformities, devices at different areas on the wafer and even devices within the chips may have inconsistent operation characteristics, or may fail altogether.
Various process parameters must be carefully controlled to ensure the high uniformity of the resulting layers. One such parameter is the temperature of the wafer or other substrate during certain steps. During CVD, for example, the deposition gases react at particular temperatures and deposit on the wafer. If the temperature varies across the surface of the substrate, uneven deposition of the reactant gas occurs and the thickness and resistivity will not be uniform. Other fabrication techniques, such as etching and annealing, also depend upon uniform temperature distribution across the substrate(s). Accordingly, it is desirable that temperature be uniform across the substrate.
Substrates can be heated using resistance heating, induction heating, or radiant heating. Among these, radiant heating is currently the favored method of supplying heat energy to a wafer in a single wafer processing chamber. Significantly, radiant heating results in short processing times and high throughput because the temperature of the wafers can be ramped up to the desired process temperature and ramped down to a satisfactory handling temperature faster than with the alternative heating techniques. Additionally, radiant heating can be controlled to maintain the wafer at the desired temperature for a sufficient duration to accomplish the process. Radiant heating energy can be supplied, for example, by banks of infrared (IR) lamps (e.g., quartz halogen lamps) above and/or below the wafer in the reaction chamber.
Radiant energy has a tendency to create non-uniform temperature distributions, including areas of less intensity (or shadows or “cold spots”) and areas of more intensity (or “hot spots”) due to the use of localized lamps and the consequent focusing and interference effects. In an effort to provide more uniform heat source distribution and a resulting uniform temperature distribution across the wafers, the industry practice has been to mount reflectors behind the bulbs to reflect the energy from the bulbs and direct the same onto the wafers. These reflectors are generally made of a base metal and are plated to efficiently reflect the light energy. However, radiant heat sources still tend to induce hot and cold spots on wafers being heated.
Temperature non-uniformities may be somewhat reduced by rotating the wafer during processing. Continuous rotation about a vertical axis about perpendicular to the wafer moves regions of the wafer that would otherwise reside within shadows or hot spots to distribute temperatures on the wafer surface somewhat more uniformly. Temperature non-uniformities may also be somewhat reduced by continuously rotating the reflector about a vertical axis as disclosed in U.S. Pat. No. 6,554,905. However, it has been found that temperature uniformity suffers even with this continual rotation of the wafer or the reflector bank about a vertical axis. Specifically, hot and cold spots remain in the form of rings at particular radial distances from the center, or in regions on the wafer surface.